Method of forming a continuous belt for a belt-type separator device

ABSTRACT

The application discloses a belt formed of first and second portions of thermoplastic sheet including, respectively, first and second pluralities of tabs spaced apart so as to define first and second pluralities of openings along corresponding first and second edges of the first and second portions of thermoplastic sheet, wherein the first plurality of tabs are joined to the second plurality of tabs so as to join the first edge to the second edge. Also disclosed is a method for forming a belt of a thermoplastic material, including forming angles on a first edge of a first portion of thermoplastic sheet and on a second edge of a second portion of thermoplastic sheet, forming first and second pluralities of openings, respectively, in the first and second portions of thermoplastic sheet, placing together the first and second edges such that they overlap, and joining the first and second portions of thermoplastic sheet together.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e) to U.S.Provisional Application Ser. No. 60/325,426 filed on Sep. 27, 2001,which is herein incorporated by reference in its entirety.

BACKGROUND

1. Field of the Invention

The present invention relates to a movable belt that may be used in abelt separation apparatus to separate a particle mixture based oncharging of the particles, and more specifically to an improved belt anda method of belt construction.

2. Discussion of Related Art

Belt separator systems (BSS) are used to separate the constituents ofparticle mixtures based on the charging of the different constituents bysurface contact (i.e. the triboelectric effect). FIG. 1 shows a beltseparator system 10 such as is disclosed in commonly-owned U.S. Pat.Nos. 4,839,032 and 4,874,507, which are hereby incorporated by referencein their entirety. One embodiment of belt separator system 10 includesparallel spaced electrodes 12 and 14/16 arranged in a longitudinaldirection to define a longitudinal centerline 18, and a belt 20traveling in the longitudinal direction between the spaced electrodes,parallel to the longitudinal centerline. The belt 20 forms a continuousloop which is driven by a pair of end rollers 22, 24. A particle mixtureis loaded onto the belt 20 at a feed area 26 between electrodes 14 and16. Belt 20 includes counter-current traveling belt segments 28 and 30moving in opposite directions for transporting the constituents of theparticle mixture along the lengths of the electrodes 12 and 14/16.

As the only moving part, the belt 20 is a critical component of the BSS.The belt 20 moves at high speed, for example, about 40 miles an hour, inan extremely abrasive environment. The two belt segments 28, 30 move inopposite directions, parallel to centerline 18, and thus if they comeinto contact, the relative velocity is about 80 miles an hour. Relatedart belts were previously woven of abrasion resistant monofilamentmaterials. These belts were quite expensive and lasted only about 5hours. The mode of failure was typically longitudinal wear stripes dueto longitudinal wrinkling, that would wear longitudinal holes in thebelt such that it would fall apart and catch on itself. The strandswould also wear where they crossed and flexed in moving through theseparator. The Applicant has made attempts to improve such belts withdifferent materials and different weaves in an attempt to find a wovenmaterial with a longer life. These attempts were unsuccessful.

Belts which are currently used in the BSS 10 are made of extrudedmaterials which have better wear resistance than the woven belts and maylast on the order of about 20 hours. The extrusion of such belts isdescribed in commonly-owned U.S. Pat. No. 5,819,946 entitled “SeparationSystem Belt Construction,” which is herein incorporated by reference.

Referring to FIG. 2, there is illustrated schematic drawing of a sectionof a belt 40 such as is currently used in the BSS of FIG. 1. Control ofthe geometry of the belt is desirable, but is difficult to achieve withextruded belts.

One example of the belt used in the BSS may comprise a structure formedof machine direction strands 42, i.e., strands that are disposed along ahorizontal length of the belt in a direction of movement of the belt(indicated by arrow 41), and cross direction strands 46, i.e., strandsthat are substantially perpendicular to the machine direction strands,as illustrated in FIG. 2. The cross direction strands 46 may be madewith a specific shape of a leading edge 43 of the belt. The machinedirection strands 46 carry the load, i.e., a mixture of constituents,and simultaneously withstand the flexing of passing over the end rollers(see FIG. 1, 22, 24) at a rate of approximately 6 rollers per second.

The extrusion process by which belts for the BSS are currently made isnecessarily a compromise of a number of factors including the choice ofthe polymer used, the choice of additives, the extrusion equipment, thetemperatures used for the extrusion process and the extrusion rate.According to one example, the operation of the extrusion process for thecurrent manufacture of extruded belts is as follows. A proper mix of abase polymer and additives (preferably pre-compounded together) is fedinto an extrusion machine, where the mechanical action of a screws heatsthe material to a temperature where it is plastic, and the extrusionmachine moves the plastic down a barrel and into a die. The die has acircular cross section, and has a number of grooves parallel to an axiswhich corresponds to the continuous machine direction strands 42. Eachcross direction strand 46 is produced by moving an inner part of the dieso that a circumferential groove which is filled with material emptiesand so forms the cross direction strand 46. Control of the geometry ofthe belt is mostly accomplished by adjusting the instantaneous extrusionrate during the formation of each individual cross direction strand 46.Material that ends up in the cross direction strand is not available forthe machine direction strand and vice versa. It may be difficulttherefore, to avoid changes in the machine direction strand crosssection while changing the extrusion rate to adjust the cross strandgeometry. After the web of machine direction strands and cross directionstrands is formed as a circular section, it is cooled, for example,through immersion in a water bath and slit and flattened to form a flatweb.

Fatigue strength is an important aspect of the belt to be used in a BSS.For good fatigue strength, stress concentrations at changes in crosssection of the strand should be avoided. Maintaining uniformity of crosssection is difficult however, and thus fatigue life of extruded belts isoften problematic.

Conveyer belts are widely used for conveying materials, and conventionalconveying belts are well developed. Usually conveyor belts areconstructed of an elastomeric material with reinforcing cords of fabric.A usual practice is to use continuous solid belts without perforations.Such belts are not suitable for the present application because of theneed for material to pass through the belt in the BSS.

Control of the belt geometry is also important as is described incommonly-owned U.S. Pat. No. 5,904,253, also herein incorporated byreference. Referring to FIG. 3, which is an enlarged portion of the BSSof FIG. 1, the directions of the counter-travelling belt segments 28, 30are shown by arrows 34 and 36, respectively. As illustrated in FIG. 3,one example of a desired geometry of the belt 40, is that of an acuteangle 44 on the leading edge 43 (see FIG. 2) of the cross directionstrands 46.

In the current practice of extrusion, the geometry of the leading edgeis controlled by adjusting the polymer composition, the additives used,and the extrusion conditions. Changing these parameters also has effectson the other properties of the belt and on its performance in the BSS.In addition, in an extrusion process, the polymers that can be used tomake such belts are limited. There are a number of polymers that cannotbe extruded and so are not options for belt manufacture by extrusion. Inaddition, large amounts of extrusion additives are needed to achievedesired belt properties through an extrusion process. However, thepresence of many additives complicates the extrusion process and canpose compatibility problems, especially for food grade applications.Many of the additives needed for dimension control also act asplasticizers and increase the rate of creep and decrease wear resistanceof the belt. Often changing one property in one way will have an adverseeffect on other properties.

Thus known methods of manufacture of belts for BSS are subject to thelimitations of the extrusion process, which limits the materials whichcan be used for belt construction, and compromises the geometry that canbe obtained. Current belts do not have the desired long wear life, goodfatigue strength, and ease of manufacture that is desired.

SUMMARY OF THE INVENTION

According to one embodiment, a method for joining, to each other, afirst edge of a first thermoplastic sheet and a second edge of a secondthermoplastic sheet, comprises acts of forming substantially matchingangles on the first edge of the first thermoplastic sheet and on thesecond edge of the second thermoplastic sheet, forming openings in thefirst edge of the first thermoplastic sheet, the openings extendingtransversely from the first edge into the first thermoplastic sheet, andforming openings in the second edge of the second thermoplastic sheet,the openings extending transversely from the second edge into the secondthermoplastic sheet. The method further comprises acts of placing thefirst and second edges together with a slight overlap, pressing thefirst and second edges together; heating the first and second edges toabove a melting temperature of the thermoplastic sheets, maintainingcontact between the first and second edges for a predetermined period oftime, and cooling the first and second edges, so that they are joinedtogether.

According to another embodiment, a method for forming a belt of athermoplastic material comprises forming angles on a first edge of afirst portion of thermoplastic sheet and on a second edge of a secondportion of thermoplastic sheet. The method also comprises forming afirst plurality of openings in the first portion of thermoplastic sheet,the openings extending transversely to a first edge of the first portionof thermoplastic sheet, and forming a second plurality of openings inthe second portion of thermoplastic sheet to be joined to the firstportion of thermoplastic sheet, the openings extending transversely to asecond edge of the second portion of thermoplastic sheet. The methodfurther comprises placing together the first and second edges of thefirst and second portions of thermoplastic sheet, such that the firstand second portions of thermoplastic sheet include overlapping portions,and joining the first and second portions of thermoplastic sheettogether.

According to yet another embodiment, a belt comprises a first portion ofthermoplastic sheet comprising a first plurality of tabs spaced apart soas to define a first plurality of openings along a first edge of thefirst portion of thermoplastic sheet. The belt also comprises a secondportion of thermoplastic sheet comprising a second plurality of tabsspaced apart so as to define a second plurality of openings along asecond edge of the second portion of thermoplastic sheet, wherein thefirst plurality of tabs are joined to the second plurality of tabs so asto join the first edge to the second edge.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features, objectives and advantages of thepresent invention will be apparent from the following description withreference to the accompanying figures in which like reference numeralsindicate like elements throughout the different figures. In the figures,which are provided for purposes of illustration only and are notintended as a definition of the limits of the invention,

FIG. 1 is a diagram of one example of a belt separator system (BSS);

FIG. 2 is an enlarged diagram of a portion of an extruded belt used in aBSS;

FIG. 3 is an enlarged view of a portion of a BSS including twoelectrodes and belt segments;

FIG. 4 is a diagram of a portion of two sections of belt to be joinedtogether, according to an embodiment of the invention;

FIG. 5 is a side view of the two sections of belt to be joined together,according to an embodiment of the invention;

FIG. 6 is a flow diagram of one example of a method for manufacturing abelt according to aspects of the invention;

FIG. 7 is an end view of two sections of belt to be joined together,according to aspects of the invention; and

FIG. 8 is a plan view of a portion of a belt according to aspects of theinvention.

DETAILED DESCRIPTION

Certain materials, such as thermoplastic materials that containpolymerization products of at least one olefinic monomer, thermoplasticsand thermoplastic elastomers are materials that have properties suitedto BSS belts. One example of a potentially useful thermoplastic materialis nylon, another is ultra high molecular weight polyethylene (UHMWPE).UHMWPE is one example of an excellent material which has properties thatmake it ideal for BSS belts. It is extremely abrasion resistant, e.g.,about an order of magnitude more resistant than the next best materialin similar service, it has a low coefficient of friction, is non-toxic,is an excellent dielectric, and is readily available. Unfortunately itcannot be extruded and so belts cannot be manufactured of it using knownextrusion techniques.

UHMWPE melts at 138 degrees Celsius. The melting point is determinedoptically when the opaque white material becomes completely clear. Theviscosity of melted UHMWPE is so high that it does not flow when melted,and articles retain their shape even when completely melted. The extremeviscosity of UHMWPE when molten results in considerable delay in theformation of crystalline domains on cooling of the molten UHMWPE, andthus the crystallization of UHMWPE is not instantaneous. Like allpolymer materials, UHMWPE has a high coefficient of thermal expansion.It also expands considerably on melting. This expansion and contractionduring and after thermal cycling presents substantial difficulties inthe thermal processing of UHMWPE. Conventional mold materials ofconstruction, such as metals, have much lower thermal expansion thanUHMWPE. Consequently, shrinkage during cooling of UHMWPE sets upsignificant thermal strains between the mold materials, UHMWPE sectionsat different temperatures, and even between UHMWPE sections withdifferent degrees of crystallinity. The degree of crystallinity is afactor in determining the density and hence the volume of any particularportion of a UHMWPE part.

According to one example of a method of manufacturing UHMWPE articles,UHMWPE is synthesized as a powder. The powder may be compression molded,at high temperature and pressure, into thick billets which may beskived, while hot, into sheets of the desired thickness. UHMWPE ismolded as thick billets because the gradients in temperature,crystallinity, density and hence specific volume are small, relative tothe dimensions of the billet, leading to small thermal strains. In athick billet, the ratio of the stress on the molded surface to thecontraction stress of the bulk of the material is relatively low. Bycontrast, thin sections have a higher ratio and are more likely toeither fail through cracking or to yield asymmetrically, resulting inbuilt-in stresses.

According to one example, BSS belts are thin, for example, on the orderof ⅛ of an inch and are about 45 inches wide. A length of material usedto form a BSS belt may be approximately 60 feet. Sheets of UHMWPE arecommercially available in sheet sizes of 4 feet by 8 feet or 4 feet by10 feet. Thus, a BSS belt may be formed by joining together several suchsheets, as is discussed below in more detail. Alternatively, a BSS beltmay be formed of a single sheet of UHMWPE, the ends of which may bejoined together to form continuous belt. In yet another example, severalnarrow sheets may be joined along a length of the sheets to form a widecomposite sheet, the ends of which may then be joined together to form acontinuous belt.

Welding, or joining, together of pieces of UHMWPE is not practiced toany significant extent in the related art, largely because of thedifficulty of dealing with the thermal strains that result. Thus whileUHMWPE is widely used for abrasion protection of steel surfaces, it isused as individual sheets which are mechanically fastened to theprotected steel surface. When conventional heat sealing type equipmentis used to attempt the welding of UHMWPE using techniques that aresuitable for other polymers, the results are not satisfactory. The weldzone becomes liquid, indicated by it becoming clear, and two liquidpieces will adhere if pressed together. However, when the article iscooled the heat-affected zone contracts substantially which results insubstantial warping of the sheet. The warping increases as the articlecontinues to crystallize, and often sheets will crack as they cool. Forexample, the heated material accommodates the thermal strain bydeforming plastically when it is hot. Then, as the cooling materialcontracts, it becomes too stiff to deform plastically and so it eitherwarps or cracks. Sheets can be seen to be flat immediately after removalfrom a welding device and cooled to room temperature, only to warp a daylater due to continued crystallization and shrinkage.

The stiffness of UHMWPE is also a sensitive function of the degree ofcrystallization. Less crystalline material is softer and has a lowermodulus. However, as the belt for a BSS is operated, the material isflexed many times a second. This flexing has a tendency to cause thematerial of the belt to further crystallize, resulting in furtherdimensional and stiffness changes.

Belts for a BSS move at high speed, for example, on the order of 20meters per second, through a narrow gap. At this speed, the belt can bequickly destroyed if it catches on something or hits a piece of trampmaterial. Warping of the belt which causes it to deviate from the planeof the electrodes is unacceptable because the belt then pushes againstthe electrode and the other segment of belt traversing between theelectrodes of the BSS, which increases the load and also can lead to thebelt “catching” on itself or on the openings in the electrode where thefeed is introduced. The belt “catching” can result in a catastrophicfailure of the belt. The belt may also become completely severedlongitudinally into two independent pieces. When the two remainingsegments of belt continue to operate in the BSS, an undesirablesituation is created because there is a stagnant stationary regionbetween the two moving pieces where conductive material can build up andcause a shorting of the high voltage electrodes.

In order to avoid the belt catching on the openings, warping of the beltmust be kept to less than half the width of the gap 31 (see FIG. 3)between the electrodes 12, 16. Applying tension to the belt might bethought to straighten out any warp. However, virtually every materialwill warp if sufficient tension is applied. All materials have a certainPoisson's ratio which requires that when a material is stretched in onedirection it contracts in all transverse directions. For example, a thinbelt material cannot support this compressive load across its width andso it buckles, resulting in longitudinal wrinkles. One failure mode thathas been observed in certain woven belts is longitudinal wrinklingleading to the parts of the belt that protrude being worn away. Wearingaway of warped sections of the belt is not acceptable in most BSSapplications.

In theory, heating and cooling entire belt sections at a time might makewelding the belt sections together possible. In practice however eventhat approach is problematic. What causes the warping is gradients ofthermal expansion leading to differential thermal strains leading todifferential stresses in the material. Thermal expansion of the materialis due to both the temperature change and to the phase change. The phasechange is not entirely uniform and isotropic. Thus a uniform temperatureapplied to the entire belt sections would not necessarily produce equalexpansion and contraction of the material. Above the melting point thematerial is viscoelastic, where the stress depends on the strain rate.In addition, heating and cooling entire belt sections at one time wouldrequire a very large mold and because the belt is desirably quite thin,the belt would likely crack when cooled in contact with a rigid metalmold.

The warping that occurs when welding two sheets of material togetherderives from irreversible deformation that occurs during the heating andcooling cycle. UHMWPE must be heated to well above the melting point toachieve sufficient mobility for the surface molecules to interdiffuseand form a strong bond upon cooling. The UHMWPE expands during heating,the total volume change being on the order of 10%, and the yield stressof the hot material is much lower than the unheated material. Coolmaterial near the heat affected zone restrains the hot material whichyields. As the hot material cools, it shrinks, and as it becomes coolerand stiffer the yield stress increases and it is able to exertsufficient stress on the unheated material to cause deflection ordeformation. Making the welded zone thinner causes the accumulatedstress in the heated material during cooling to exceed the strength ofthe cooling material and it fails by cracking. Making the weld very thinalso reduces the strength of the weld.

Deformation or warp of the belt made from UHMWPE is determined by thecontraction of the heat affected zone and the buckling of thesurrounding material. The amount of any warpage is dependent on thetotal strain, which depends on the total length of the weld. Forexample, in a 40 inch wide belt, a 10% strain (resulting from a 10%change in volume as discussed above) results in plus 2 inches ofdeformation- for cold material and minus 2 inches of deformation for hotmaterial. There is some yielding of the hot material, but even a 2.5%length change (1 inch in 40) results in substantial warpage. The warpageout of the plane of the belt may be a critical parameter for BSS belts,and depends on the wavelength of the warp. If the warp is taken up as asingle sine wave, the total out of plane deformation can be calculatedapproximately by: $\begin{matrix}{d^{2} = {\left( {\frac{\lambda}{4}*1.025} \right)^{2} - \left( \frac{\lambda}{4} \right)^{2}}} & (1)\end{matrix}$

where d=deformation and λ=wavelength.

Thus, if the wavelength of the sine wave is 80 inches (twice the lengthof the 40 inch weld), equation 1 yields a total deformation, d, of 4.5inches. This is far too much to be accommodated in most systems, becauseif, in order to avoid the belt catching on openings as discussed above,warping of the belt must be kept to less than half the width of the gapbetween the electrodes, a deformation of 4.5 inches means that the gapwidth between the electrodes should be at least 9 inches. This is toowide a separation of the electrodes for efficient operation of the BSS.By contrast, if the same percentage strain is taken up with a warpagewavelength of 2 inches, the out of plane deformation, d, given byequation 1 is now 0.1 inches. This amount is less than the usual gapbetween the electrodes of the BSS. In practice much of this deformationis taken up plastically and elastically so the actual warpage may bemuch less than 0.1 inches.

As mentioned above, the wavelength of the deformation determines themagnitude of the out of plane protrusion of the belt/sheet. The part ofthe sheet that experiences compressive thermal strain buckles becausethe compression load is greater than the critical load that can beresisted without buckling. The critical load that produces buckling islowest at the longest wavelength deformation and increases rapidly asthe wavelength decreases. This critical load can be calculated usingEuler's column formula: $\begin{matrix}{P_{c\quad r} = {\pi^{2}*E*\frac{A}{L^{2}}}} & (2)\end{matrix}$

where E is the modulus of the material, A is the moment of inertia ofthe column and L is the length of the weld.

Strain accumulates between the heat affected and non-heat affected zonesof welded sections of the belt formed of UHMWPE, and causes deformation.The wavelength of the warp deformation may be controlled by setting theboundary conditions for stress and strain to zero at the ends of theweld by creating free edges. Short welds result in a higher criticalload for buckling and at this higher load, more of the thermal strain isaccommodated through non-buckling deformation. If the welds are madeshort, all of the warp will be accommodated within the welds, and thewavelength will then be at most twice the length of the weld (one half asine wave). Thus, by making the welds short (on the order of 1 inch) theout of plane component of any warpage will be small.

Therefore, one aspect of a sheet welding method of the invention is toprovide openings, for example, cuts in the sections of, for example,UHMWPE, to be welded such that the length of weld is relatively short,and so that the heat affected zone is within the area bounded by theopenings. This allows the thermal strains to be taken up elastically inthe heat affected and non-heat affected material. For example, sheetsthat have been joined by the process of this invention may be on theorder of 10 feet long, or 120 inches. The heat affected zone is on theorder of 1.2 inches wide, or approximately 1% of the sheet length.Welding of the UHMWPE sheets under these conditions does produce holesin the belt, however in BSS's most of the belt is open area andadditional openings around a joint are not detrimental. Any warpage inthe resultant welded sheets is very small and does not protrude beyondthe plane of the belt. It is to be appreciated that individual smallsheets can be so joined to form composite sheets, and a single sheet ora composite sheet can be joined to itself to form an endless loop.

Referring to FIG. 4, there is illustrated a portion of one example ofthe edges of a sheet prepared for welding according to the presentinvention. It is to be appreciated that joining may be accomplished bythermal welding, and also by other methods of plastic welding known tothose of skill in the art, such as ultrasonic, dielectric, infrared. Asdiscussed above, openings 50 are formed in each of a first sections (orsheet) 52 and a second section (or sheet) 54 of UHMWPE that are to bejoined to form a belt. It is to be understood that the sections 52 and54 of UHMWPE may be different sheets that are to be joined together, ormay be edges of a same sheet or a composite sheet that are to be joinedto form a continuous belt. Openings 50 are made in the sheet prior tothe formation of the join. The openings 50 in the material at the joinserve two purposes. First, space is provided where the material isremoved by the cuts to accommodate the free expansion of the UHMWPE asit expands during the heating. Second, adjacent join sections 56 (tabsof material) are decoupled from each other so that the thermal strain inone section that results during cooling and contraction does not add toan adjacent section, and so accumulate along a long length of the join.Accommodating the expansion on heating and allowing contraction oncooling prevents thermal strains from accumulating across the width ofthe belt and causing warpage of the belt during the welding process.

Lines 58 and 60 demarcate the extent of the heat affected zone during ajoining process. It can be seen that the openings 50 extend past theheat affected zone so that the heat affected zone is within the areabounded by the openings. This allows the thermal strains to be taken upelastically in the heat affected and non-heat affected material, asdiscussed above. In the illustrated example, the openings have roundedsurfaces. It is desirable to prevent stress concentration at the base ofthe opening, and so it may be desirable to use a rounded cutter to formthe opening, however, other shaped openings may be used as well.According to one example, the width of the belt (sections of material tobe joined) may be approximately 40 inches, and the tabs of material 56that form the material of the weld are approximately 1 inch wide. Thewidth of the openings 50 is not critical, so long as material fromadjacent tabs 56 does not expand across the opening 50 during thewelding process and upset the stress and strain free edge boundarycondition.

Breaking the weld up into a number of shorter weld segments with openspace (i.e., the openings 50) between them, as illustrated, also has theadvantage that the open spaces act as crack terminators. Cracks readilypropagate through a solid material because the stress is concentrated atthe tip of the crack. An opening sufficiently large that the stress ofthe crack can be distributed elastically around the opening is aneffective crack terminator.

A critical parameter of BSS belts may be their uniformity of thicknessand the lack of protrusions from the surface which can catch on openingsin the electrodes or on the confronting section of the belt as the belttraverses with the BSS. As discussed above, making the joint betweensheets of a multiplicity of short welds addresses the warpage problem,but the joining procedure should also not generate protrusions. A buttweld, e.g., a weld of planar surfaces, does not have sufficient strengthto withstand normal tensile loads in an operating BSS and there is adiscontinuity in material stiffness across such a joint. During passageover the multiple rollers of the separator (at a rate of approximately 6per second), the joint is subjected to multiple cycles of positive andnegative bending. This cyclical back and forth bending results infailure of the joint in a butt weld. In contrast, a joint made by simplyoverlapping material may result in excess thickness of the joint and thebelt. Constraining the thickness by confining the weld between heatedplatens may cause the excess material to extrude out. UHMWPE does notdeform plastically in these cases, instead, the material cracks. Thecracks provide for stress concentrations which have the potential topropagate into the bulk material. Discontinuities in temperature historycan also cause discontinuities in degree of crystallinity and hencediscontinuities in material modulus. Such discontinuities in modulus canalso lead to stress concentration and cracking.

Accordingly, to avoid the above-described problems, a weld jointpreparation exhibiting tapering of the sections to be joined, may usedaccording to one embodiment of the invention. FIG. 5 illustrates across-section of a weld according to an embodiment of the invention. Asshown in FIG. 5, each of the tabs of material 56 (see FIG. 4) may betapered with an angle 70. In one embodiment, substantially matchingangles may be formed on each of the two sheets (or edges) to be joined,such that when the sheets are placed together with a slight overlap, thesubstantially matching angled edges fit together, as shown. The taperingof the join is of particular importance. This tapering allows anydiscontinuity in modulus which occurs in the welded material to bespread out over a longer space and so reduces any tendency for stressconcentration.

A large percentage of open area is desirable in a BSS belt, and a“strong” belt is also desirable. Thus, there is a need to optimize atradeoff between these two features. The strength of the welded jointdepends on the cross section of that joint. The strength of the heataffected material at the weld is lower than that of the bulk material.However, much of the bulk material is removed to provide for the openarea that is necessary for proper BSS operation. The weld therefore,need only be as strong as the weakened material of the remainder of thebelt. This may be accomplished by using a larger cross section for thewelds than for the remainder of the belt. Increasing the area of theweld allows the joint to develop the full strength of the material eventhough the weld itself has lower strength. Using a tapered joint, suchas illustrated in FIG. 5, also reduces the discontinuity in materialproperties that can lead to stress concentration and eventual failure.

Referring again to FIG. 5, the weld may be produced by machining the twoends 52, 54 to be joined in matching acute angles, as discussed above.In one example, the angle may be less than approximately 30 degrees. Thesmaller the angle the larger the cross section of the weld. The tensileload on the belt is transferred by shear through this weld. In oneexample, an angle (70) of 15 degrees has been used and has worked well.This angle increases the cross section of the joined area for thetransfer of the tensile load by shear by about 4 times the cross sectionof the unmachined material. In another example, a range of 10 to 45degrees may be used. If the angle is too large, there is limitedoverlap, and the accuracy required for the edge preparation may becomeexcessive. Similarly, when the angle gets too small, the sections becomevery thin and the weld width may become excessive.

The strength of the joint exceeds that of the bulk material even if thestrength of the weld is ⅓ that of the base material. However the jointdoes represent a weakened portion of the belt and care needs to be takenthat failure does not start at one point and propagate through fatigueto other regions. This is accomplished by ensuring that the opensegments are sufficiently open that the excess material can freelyexpand during the welding process and by ensuring that there are nosurface defects in the heat affected material such as surface crackswhich may initiate propagating fatigue cracks. If any such cracks doform during the welding process, it is desirable to machine away thecracked material before using the belt.

Referring to FIG. 6, there is illustrated a flow diagram of oneembodiment of a method for manufacturing a belt, according to aspects ofthe invention. As discussed above, in a first step 100, one or moresheets of thermoplastic sheet may be provided that are to be joinedtogether. In one example, two or more sheets may be joined to provide alarger composite sheet, that may ultimately formed into a continuousbelt. Opposing edges of either a single sheet or a composite sheet maybe joined to form a continuous belt. The following method applies toeither the joining of separate sheets or of opposing edges of a samesheet.

In a next step 202, the edges to be joined may be tapered, and theopenings 50 (see FIG. 4) formed (step 204), as discussed above. The weldof the edges may be begun to be produced, in steps 206 and 208, byorientating the two ends of the sheets 52, 54 in a welding machine whichpresses the machined ends of the sheets together with flat platens 76,78 such that the overlap, as shown in FIG. 5. The space between theplatens may be controlled by the introduction of spacer elements 72 and74, in step 210. When sufficiently rigid platens are used, the spacerscan be disposed at the ends, as shown. If less rigid platens are used,the spacers may be inserted along an interior, for example in the openspace provided by openings 50 between the tabs of material 56 (see FIG.4). The location of these spacers is illustrated in FIG. 7 which showsan end view of the sheets 52, 54 between the platens 76, 78. The spacerelements 72, 74 may have a thickness that is substantially equal to athickness of the belt, and are made of a material that does not softenat the temperatures used.

The platens 76, 78 are then closed, as indicated in step 212, andpressure is applied to the platens, and transferred through the platensto the sheets 52, 54. In a next step 214, the platens are heated eitherelectrically or more conveniently with a circulating hot fluid. Pressureis maintained on the weld during the heating and cooling cycle. In oneexample, the temperature is increased to approximately 395 F. (or 202degrees C.) and is held for about 30 minutes. The heating is thenstopped, and cooling fluid is circulated to cool the weld to nearambient temperature. The weld is cooled so that it does not deform onbeing removed from the weld machine. The belt should be kept in areasonably flat configuration for some time after the weld is made whilethe UHMWPE continues to crystallize. The glass transition temperaturefor polyethylene is 153 K. Above that temperature it will continue tocrystallize over time.

As discussed above, in one embodiment, the plastic is brought to weldingtemperature by direct contact with heated platens. Alternate methods ofheating are known, such as heating by ultrasonic or infra red radiation.Alternate methods can be used provided that the temperature of thematerial during welding is controlled and that pressure is applied toensure that the thickness of the joint is substantially equal to that ofthe parent material.

Using a tapered weld also allows the weld to be subjected to significantpressure during the welding process. Sometimes, the two pieces to bewelded do not align exactly, and there is a slight “interference” fit81, as shown in FIG. 5. During the welding process, the material is heldbetween two heated platens 76, 78. The platens provide a referencesurface and determine the thickness of the weld. Providing for overlapensures that there is sufficient material at the weld and that somematerial may flow to the open spaces provided. The degree of overlap canbe quantified by comparing the thickness of the joint before welding(dimension 80) to that of the parent material (dimension 82). The sum ofthe dimension of the parent material (82) and the overlap (81) equalsthat of the thickness before welding (80). The fractional degree ofoverlap is (80-82)/82. To express the fractional degree of overlap as apercentage, the fractional value is multiplied by 100. In one example,the overlap is approximately 10%. In another example, an overlap of 60%was used and has worked well, but other values may be used as well. Theoverlap also serves to reduce the degree of accuracy required in themachining of the mating surfaces. It may be particularly important thatthe molten surfaces be pressed together during the welding process. Ifin the machining process, there is an inaccuracy in the surfaces suchthat they are not in contact, those surfaces will not form asatisfactory weld. By providing for overlap, a single fixed flat platenand a single movable flat platen can be used to press the surfacestogether.

It is to be appreciated that the heating and cooling cycle is important,both in the temperatures reached and the time at different temperatures.It has also been found that edge effects are important in the heattransfer to and from the belt during the welding process. These edgeeffects can be overcome by using a sacrificial material at the edge ofthe belt which may later be cut off of the belt and discarded, to movethe edge effect from the belt edge into a disposable member.Conveniently this member can also be a spacer that controls the spacingof the platens to that of the desired thickness of the belt.

A potential failure mode is the unpeeling of the weld. The belt issubjected to significant shear on one surface where it contacts theelectrodes at tens of meters per second. Peeling back with wear of theexposed piece and sometimes with the protruding piece catching on a feedport can lead to catastrophic failure of the belt. The incidence of sucha failure mode may be reduced by choosing the orientation of the weldoverlap such that the thin tapered portion of the weld is on thetrailing edge of the belt. With this orientation there is no tendencyfor the edge to peel back and for a failure of the weld to initiate andpropagate across the joint. The orientation of the weld edges is seen inFIG. 5 relative to the leading edges of the cross strands 46. The beltmay be installed in the machine with surface 88 facing an electrode andsurface 90 facing another section of belt. The direction of travel ofthe belt with respect to stationary electrodes would then be as shown byarrow 92.

Producing a belt in this manner from machined sheets of UHMWPE allowsfor the profiles discussed in U.S. Pat. No. 5,904,253, hereinincorporated by reference, to be utilized. One example of a convenientmethod is to use a multi-axis machine tool. With this device, a sheet isloaded onto a table and a cutter head is moved across the sheet and eachopening in the belt may be cut individually. Through the proper choiceof cutting tool, the holes can have the desired leading edge andtrailing edge features as desired. It is to be appreciated that thedesired leading edge geometry can be obtained through forming means suchas molding, punching, machining, water jet cutting, laser cutting, andthe like.

Referring again to FIG. 6, in a step 216 of this embodiment of themethod of manufacturing the belt, the total length of the joinedsections may be evaluated to determine whether it is sufficiently longto form a complete belt for the desired application. If not, additionalsheets may be welded by repeating steps 208-214 as indicated by step218, to form a composite sheet of a desired length. Opposing edges ofthe composite sheet may then be joined together to form a continuousbelt, as indicated in steps 220-224.

The belt manufacturing method disclosed herein can also be utilized toproduce belts for other applications. In many other applications, holesin the belt may be undesirable. As described above, according to oneembodiment, material at the weld may be removed to break-up the weldinto short independent sections. After this is done and the weld ismade, the holes can be filled in with material to give a hole-free belt.It may be desirable, however, to allow for stress distribution aroundthe welds and for the welds to remain structurally independent. One wayof doing this is to fill the holes with a low modulus material, such asthin polyethylene film or foam. The foam is easily deformed and willaccommodate substantial thermal strains generated during the welding.

With the capability of welding sheets of UHMWPE into continuous endlessbelts, flexibility in belt geometry can be achieved. The sheets can beheld on a table and holes can be machined in the sheet. There iscomplete flexibility in selecting the geometry of the cross directionstrands and the machine direction strands. The machine direction strandscan be designed to have excellent fatigue life and the cross directionstrands can have excellent separation geometry. The method ofmanufacture and materials described herein can thus be used to achievelonger life belts which are amenable to better geometry control.Producing BSS belts in this manner also allows for additional featuresto be incorporated.

It is to be appreciated that the BSS belt is used in a difficultenvironment. Flyash is abrasive and is often filled with tramp material.Stones, welding rod, bolts, gloves, refractory, and all manner of trampmaterial has been found in flyash, and numerous belt failures haveresulted from tramp material. If the foreign object is larger than thegap between the electrodes, the object will not enter the machine butwill remain hung up at a feed point until it gets ground up or until thebelt is destroyed. In one embodiment of a belt of the invention,periodic strong transverse elements may be provided in the belt. Anillustration of a portion of a belt showing such strength elements 100,101, 102, 103 is shown in FIG. 8. The belt can get hung up at one ofthese strong elements and be stopped so that the machine can be openedand cleared of the tramp material. According to one example, the strongelements may be provided by periodically omitting machining holes 106 inthe belt. Often it is useful to have this increased strength segment 100as part of the weld. The belt may be seen to be torn in the lengthwisedirection until the tear reaches a weld where the tear is terminated.Belts can then survive several such events occurring at differentpositions on the belt whereas with prior belts, a single event wouldresult in a lengthwise tear the entire length of the belt and so destroyit. It is to be appreciated that these imporforate regions can begrouped as in a line either lengthwise, for example, along an edge 104.Alternatively, a strength member 101 may be provided as an imporforatesection across a width of the belt, or diagonally (e.g. region 102, orthey can be randomly disposed (e.g. regions 103), or disposed in aregular pattern.

Having thus described various illustrative embodiments and aspectsthereof, modifications, and alterations may be apparent to those ofskill in the art. For example, the sheet welding method disclosed hereinmay be used to weld materials other than UHMWPE, such as high densitypolyethylene nylon, polyester, and that thermoplastic sheet includesboth perforated and imporforate sheets of any thermoplastic material.Such modifications and alterations are intended to be included in thisdisclosure, which is for the purpose of illustration and not intended tobe limiting. The scope of the invention should be determined from properconstruction of the appended claims, and their equivalents.

1. A method for joining, to each other, a first edge of a firstthermoplastic sheet and a second edge of a second thermoplastic sheet,the method comprising acts of forming substantially matching angles onthe first edge of the first thermoplastic sheet and on the second edgeof the second thermoplastic sheet such that the first and second edgesare inclined with respect to an upper surface of the first and secondthermoplastic sheets, respectively, thereby forming first and secondmatching inclined edges; forming openings in the first edge of the firstthermoplastic sheet, the openings extending transversely from the firstedge into the first thermoplastic sheet; forming openings in the secondedge of the second thermoplastic sheet, the openings extendingtransversely from the second edge into the second thermoplastic sheet;placing the first and second matching inclined edges together with aslight overlap such that the substantially matching inclined edges fittogether; pressing the first and second matching inclined edgestogether; heating the first and second matching inclined edges to abovea melting temperature of the thermoplastic sheets; maintaining contactbetween the first and second matching inclined edges for a predeterminedperiod of time; cooling the first and second matching inclined edges, sothat they are joined together; and wherein the openings extendsubstantially beyond a heated zone of the first and second matchinginclined edges created by the heating act.
 2. The method as claimed inclaim 1, wherein the act of pressing the first and second matchinginclined edges together comprises pressing the first and second matchinginclined edges together with a pair of platens.
 3. The method as claimedin claim 2, wherein a width of each platen of the pair of platens isapproximately 1.5 inches.
 4. The method as claimed in claim 1, furthercomprising removing the joined first and second edges matching inclinedfrom between the platens.
 5. The method as claimed in claim 1, whereinthe act of joining together a first edge of a first thermoplastic sheetand a second edge of a second thermoplastic sheet includes joiningtogether a first edge and a second edge of a same thermoplastic sheet,to provide a continuous belt.
 6. The method as claimed in claim 1,further comprising joining the first and second sheets with at least oneadditional thermoplastic sheet to form a composite sheet.
 7. The methodas claimed in claim 6, further comprising joining together opposingedges of the composite sheet, to form a continuous belt.
 8. The methodas claimed in claim 1, wherein the act of heating the first and secondmatching inclined edges includes heating by direct contact with heatedplatens.
 9. The method as claimed in claim 1, wherein the act of coolingthe first and second matching inclined edges includes cooling by directcontact with cooled platens.
 10. The method as claimed in claim 1 wherethe thermoplastic material contains a polymerization product of at leastone olefinic monomer.
 11. The method as claimed in claim 1, wherein thefirst and second thermoplastic sheets compriseUltra-high-mololecular-weight polyethylene.
 12. The method as claimed inclaim 1, wherein the act of placing together the first and secondmatching inclined edges includes placing together the first and secondinclined edges so as to form an overlap of the first and secondthermoplastic sheets, the overlap having a thickness that isapproximately 10% greater than a thickness of the first thermoplasticsheet.
 13. A method for forming a belt of a thermoplastic material, themethod comprising: tapering a first edge of a first portion ofthermoplastic sheet and a second edge of a second portion ofthermoplastic sheet so as to form substantially matching angled edges onthe first and second portions of thermoplastic sheet; forming a firstplurality of openings in the first portion of thermoplastic sheet, theopenings extending transversely to a first edge of the first portion ofthermoplastic sheet; forming a second plurality of openings in thesecond portion of thermoplastic sheet to be joined to the first portionof thermoplastic sheet, the openings extending transversely to a secondedge of the second portion of thermoplastic sheet; placing together thefirst and second edges of the first and second portions of thermoplasticsheet, such that the first and second portions of thermoplastic sheetinclude overlapping portions and the substantially matching angled edgesfit together; and joining the first and second portions of thermoplasticsheet together; and wherein the first plurality of openings extendtransversely to the first edge of the first portion of thermoplasticsheet, beyond the overlapping portions, and wherein the second pluralityof openings in the second portion of thermoplastic sheet extendtransversely to the second edge of the second portion of thermoplasticsheet, beyond the overlapping portions.
 14. The method as claimed inclaim 13, wherein the act of joining includes an act of welding thefirst and second portions of thermoplastic sheet together.
 15. Themethod as claimed in claim 13, wherein the act of tapering includesforming substantially matching acute angles on the first edge and on thesecond edge.
 16. The method as claimed in claim 13, wherein the act ofjoining comprises: heating under pressure at least the overlappingportions of the first and second portions of thermoplastic sheet toabove a melting temperature of the first and second portions ofthermoplastic sheet so that the overlapping portions of the first andsecond portions of thermoplastic sheet become joined together; andcooling at least the overlapping portions of the first and secondportions of thermoplastic sheet.
 17. The method as claimed in claim 13,wherein the thermoplastic material isultra-high-molecular-weight-polyethylene.
 18. The method as claimed inclaim 13, further comprising perforating the first and second portionsof thermoplastic sheet.
 19. The method as claimed in claim 18, whereinthe act of perforating the first and second portions of thermoplasticsheet includes perforating the first and second portions ofthermoplastic sheet such that an open area of perforated sheet exceeds50% of a total area of the perforated sheet.
 20. The method as claimedin claim 18, wherein the act of perforating the first and secondportions of thermoplastic sheet includes forming perforations so as toproduce a leading edge of the belt with an acute angle.
 21. The methodas claimed in claim 20, wherein the act of perforating includes formingperforations so as to produce the leading edge of the belt with theacute angle being less than approximately 60 degrees.
 22. The method asclaimed in claim 13, wherein the act of placing together the first andsecond portions of thermoplastic sheet includes placing together thefirst and second portions of thermoplastic sheet such that theoverlapping portions have a thickness that is approximately 10% greaterthan a thickness of the first portion of thermoplastic sheet.
 23. Themethod as claimed in claim 13, wherein the acts of forming openingsinclude forming the openings such that a spacing between the openings isapproximately 1 inch.
 24. The method as claimed in claim 13, wherein theacts of forming openings include forming openings having a width greaterthan approximately ⅛ inches.
 25. The method as claimed in claim 13,wherein the acts of forming openings include forming openings having alength of approximately 2 inches.
 26. The method as claimed in claim 13,wherein the thermoplastic sheet is nylon.
 27. The method as claimed inclaim 13, wherein the thermoplastic material comprisesUltra-high-molecular-weight polyethylene.